In a hybrid structure for a construction machine such as a power shovel, for example, power is generated by rotating a power generator using a surplus output of an engine, the generated power is stored in a battery, and an actuator is operated by driving an electric motor using the power of the battery. Further, power is generated by driving a hydraulic motor to rotate the power generator using a discharge energy of the actuator, whereupon the generated power is likewise stored in the battery and the actuator is operated by driving the electric motor using the power of the battery (see JP2002-275945A).